Spatial light modulation (SLM) display systems are visual display systems that are used as an alternative to conventional cathode-ray tube (CRT) systems. SLM systems are used in a variety of applications such as television screens. One type of SLM may be referred to as a projection display system. Due to their superior clarity and performance, they are often used in high-end applications such as high-definition television (HDTV). Projection display systems transmit light produced by one or more light sources to create the display on a display screen. One popular projection display system is commercially available as DLP® (or Digital Light Processing®). DLP® utilizes a digital micromirror device (DMD), an array of thousands of tiny mirrors to properly reflect light from the light source to produce the image for display. One example of a DMD-based projection display system is illustrated in FIG. 1.
FIG. 1 is a simplified configuration diagram illustrating selected components of an exemplary projection display system 10. The display system 10 includes various components that define an optical path 5 between light source 11 and display screen 19. Light source 11 may be, for example, an ultra-high pressure (UHP) arc lamp. Display screen 19, which may be an HDTV screen, presents the visual image display intended to be seen by the viewer. The remaining components are mounted within an appropriate housing (not shown). In operation, light emitted from the light source 11 is applied through a first condenser lens 12 and then through a rotating color wheel 13. Color wheel 13 will typically rotate at least once per frame (of the image to be displayed). The light passing through the color wheel 13 next passes through a second condenser lens 14 before illuminating DMD chip 15. It is chiefly DMD chip 15 that modulates the light traveling through optical path 5 to produce a visual image.
To accomplish this, the DMD chip 15 includes an array of tiny mirror elements, or micromirrors (typically on the order of one million of them). Each mirror element is separately controllable. For example, they may be mounted on a torsion hinge and support post above a memory cell of a CMOS static RAM as shown in FIG. 2. FIG. 2 shows a portion of a typical DMD chip 15 having mirror elements 21 suspended over a substrate 23. Electrostatic attraction between the mirror 21 and an address electrode 25 causes the mirror to twist or pivot, in either of two directions, about an axis formed by a pair of torsion beam hinges 27a and 27b. Typically, the mirror rotates about these hinges until the rotation is mechanically stopped. The movable micromirror tilts into the on or off states by electrostatic forces depending on the data written to the cell. The tilt of the mirror is on the order of plus 10 degrees (on) or minus 10 degrees (off) to modulate the light that is incident on the surface.
The DMD's are controlled by electronic circuitry (not shown) that has been fabricated on the silicon substrate 23 and is generally disposed under the DMD mircromirror array. The circuitry includes an array of memory cells (also not shown), typically one memory cell for each DMD element, connected to the address electrodes 25. The output of a memory cell is connected to one of the two address electrodes and the inverted output of a memory cell is connected to the other address electrode.
The operation data is provided by a timing and control circuit 17 as determined from signal processing circuitry according to an image source 16 (as shown in FIG. 1). Once data is written to each memory cell in the array, a voltage is applied to the individual DMD mirrors 21 creating a large enough voltage differential between the mirrors 21 and the address electrodes 25 to cause the mirror to rotate or tilt in the direction of the greatest voltage potential. Since the electrostatic attraction grows stronger as the mirror is rotated near an address electrode, the memory cell contents may be changed without altering the position of the mirrors once the mirrors are fully rotated. Thus, the memory cells may be loaded with new data while the array is displaying previous data.
As should be apparent, the rotation of the individual mirror elements 21 determines the amount and quality of light that will be directed at lens 18. The light reflected from any of the mirrors may pass through a projection lens 18 in order to create images on the screen 19. The intensity of each color displayed on the screen 18 is determined by the amount of time the mirror 21 corresponding to a particular pixel directs light toward screen 31. For example, each pixel may have 256 intensity levels for each color (e.g., red, green or blue). If the color level selected for a particular pixel at a particular time is 128, then the corresponding mirror would direct light toward that area of screen 31 for ½ (e.g., 128/256) of the frame time.
Using multiple arrays of LEDs is also an option for illuminating the DMD 15 as shown in FIG. 3. FIG. 3 is a simplified configuration diagram illustrating selected components of an exemplary optical path 20. As with the example of FIG. 1, optical path 20 is part of a projection display system (although the projection lens and the display screen are not shown in FIG. 3). Exemplary optical path 20 of FIG. 3 is a “fixed array” system, having three stationary arrays; red array 28, green array 30, and blue array 32. No moving parts, such as color wheel 13 shown in FIG. 1, are needed. The light is applied sequentially by turning on and off each of the red, green, and blue arrays. One advantage of using three LED arrays rather than a single arc lamp in a projection display system is that when one LED array is on, the other two are off. This is an advantage because it means that when a given LED array is on nearly all of the light collected by the optics for illuminating the DMD is within the usable spectrum that the optics will pass to the DMD. In this way it is more efficient.
In operation, light from blue LED array 32 is transmitted via lens 33 through filter 34 and filter 35 to optical integrator 36. Likewise, light from green LED array 30 is passes through lens 31 and then is reflected from filter 34 but then transmitted through filter 35 to optical integrator 36. Light from red LED array 26 is reflected from filter 35 to optical integrator 36. Light from optical integrator 36 is transmitted to (and through) relay lenses 37 and 38, from where it is directed to DMD array 15. Light from DMD array 15 is then selectively directed to a projection lens (Not shown) and on to a screen or other display medium (also not shown).
For another example, in an arrangement that may be used in conjunction with the optical path illustrated in FIG. 3 (or a similar system), light from an array of LEDs may be collimated into a single light pipe. These LEDs may be narrow-spectrum or wide spectrum or both. LEDs emitting light at different wavelengths may also be present in the same array. One optical path for performing this function is shown in FIG. 4. Note that as used herein, the term ‘optical path’ may denote all of the optical components in a display system, or only a selected portion of them. Note also that while the ends of the optical path are established by the light source (or sources) and the visual image display screen, these components may also be considered a part of the optical path. FIG. 4 is a simplified representation of an optical path 40 for collimating the light emitted from a light source, in this case an array of LEDs 41. To illustrate the manipulation of the light beam by the optical-path components, in FIG. 4 (as in other Drawings), it is represented by a number of lines that are being altered by each component. This representation is for convenience, and while approximately correct, it is not meant to connote an exact light path or relative intensity.
The array of LEDs 41 is positioned so that the emitted light is in substantial part received by a convex lens 42, which reduces the light beam propagation angle significantly. A second lens 43, oriented in opposing fashion at a distance d1 from the first convex lens 42, directs its light beam into light pipe 44. Light pipe 44 further collimates the light into a narrow beam that may be directed toward, for example, a DMD such as that shown in FIG. 1. Note that here, as with any optical path in a projection display system, it is important that the components in the optical path facilitate the transmission of light as efficiently as possible.
As should therefore be apparent from FIG. 4, LED array 41 may be limited in size because a substantial portion of the light emitted from it must be received by convex lens 42, wasted light energy being undesirable in display systems. In addition, the individual LEDs of LED array 41 must be positioned relatively close together. FIG. 5 is a schematic representation of an LED array 41 that might be used in conjunction with optical path 40 of FIG. 4. In the exemplary representation of FIG. 5, LED array 41 includes LEDs 51 through 56. LEDs 51 through 56 are mounted in close proximity to each other on substrate 50. Although this arrangement may produce satisfactory illumination for transmitting along optical path 40 (shown in FIG. 4), the close proximity of LEDs 51 through 56 means that the electrical power and control (data) connections may be difficult to route.
Moreover, the heat load building up on substrate 50 may be difficult to dissipate adequately. And while an LED fixed array is in some ways more efficient, however, a small amount of overlap occurs in the distribution of emitted light spectrum between, for example, the green and blue LEDs. In the optics arrangement of FIG. 3, the color filters cannot pass the overlapping colors for both the green and blue LED arrays. The “tail” of the spectrum for both green and blue is rejected by the color filters 34 and 35. But this rejection of light by the optics is not as pronounced as in a color wheel based projector using an arc lamp. So, in general, an LED fixed array based projector is less wasteful in terms of rejecting illumination source light because of its spectrum.
LED technology, however, has lagged behind arc lamp technology in being able to achieve comparable screen lumens. LED arrays can be used in a projector but not enough LEDs can fit into the limited etendue (light collection capacity) of the DMD so that acceptable screen lumens can be achieved in the marketplace. Power to each LED in any array can be increased but limitations are reached in allowable LED junction temperatures. In other words, the buildup of heat may become a problem and limit the number of LEDs that can be used in the light source array or arrays.
Needed therefore, is a display system that can efficiently produce adequate illumination for the creation of a visual image. The present invention provides just such a solution.